A fuel cell is an electrochemical cell comprising two electrodes separated by an electrolyte. A fuel, e.g. hydrogen, an alcohol such as methanol or ethanol, or formic acid, is supplied to the anode and an oxidant, e.g. oxygen or air, is supplied to the cathode. Electrochemical reactions occur at the electrodes, and the chemical energy of the fuel and the oxidant is converted to electrical energy and heat. Electrocatalysts are used to promote the electrochemical oxidation of the fuel at the anode and the electrochemical reduction of oxygen at the cathode.
Fuel cells are usually classified according to the nature of the electrolyte employed. Often the electrolyte is a solid polymeric membrane, in which the membrane is electronically insulating but ionically conducting. In the proton exchange membrane fuel cell (PEMFC) the membrane is proton conducting, and protons, produced at the anode, are transported across the membrane to the cathode, where they combine with oxygen to form water.
A principal component of the PEMFC is the membrane electrode assembly (MEA), which is essentially composed of five layers. The central layer is the polymer ion-conducting membrane. On either side of the ion-conducting membrane there is an electrocatalyst layer, containing an electrocatalyst designed for the specific electrolytic reaction. Finally, adjacent to each electrocatalyst layer there is a gas diffusion layer. The gas diffusion layer must allow the reactants to reach the electrocatalyst layer and must conduct the electric current that is generated by the electrochemical reactions. Therefore the gas diffusion layer must be porous and electrically conducting.
Electrocatalysts for fuel oxidation and oxygen reduction are typically based on platinum or platinum alloyed with one or more other metals. The platinum or platinum alloy catalyst can be in the form of unsupported nanoparticles (such as metal blacks or other unsupported particulate metal powders) or can be deposited as even higher surface area particles onto a conductive carbon substrate or other conductive material (a supported catalyst).
Conventionally, the MEA can be constructed by a number of methods outlined hereinafter:
(i) The electrocatalyst layer may be applied to the gas diffusion layer to form a gas diffusion electrode. A gas diffusion electrode is placed on each side of an ion-conducting membrane and laminated together to form the five-layer MEA;
(ii) The electrocatalyst layer may be applied to both faces of the ion-conducting membrane to form a catalyst coated ion-conducting membrane. Subsequently, a gas diffusion layer is applied to each face of the catalyst coated ion-conducting membrane.
(iii) An MEA can be formed from an ion-conducting membrane coated on one side with an electrocatalyst layer, a gas diffusion layer adjacent to that electrocatalyst layer, and a gas diffusion electrode on the other side of the ion-conducting membrane.
Typically tens or hundreds of MEAs are required to provide enough power for most applications, so multiple MEAs are assembled to make up a fuel cell stack. Flow field plates are used to separate the MEAs. The plates perform several functions: supplying the reactants to the MEAs; removing products; providing electrical connections; and providing physical support.
In normal fuel cell operation where the cathode is promoting the oxygen reduction reaction and the anode is promoting the hydrogen oxidation reaction, the electrode potentials are typically 0.9-0.6 V and 0.0-0.1 V vs a standard hydrogen electrode, such as a reversible hydrogen electrode (RHE), respectively. However, in a number of real-life operational situations other reactions may be promoted intermittently at either the anode or cathode, and these can occur at undesirably high electrochemical potentials of above 1.0 V, or even above 2.0 V, at either the anode or cathode. These elevated potentials can cause irreversible damage to the electrocatalyst layer/electrode structure, due to corrosion of any carbon present in the layer (such as the support material for the catalyst) and loss of active surface area of the nanoparticulate electrocatalyst metal due to various metal sintering degradation mechanism that occur during high potential excursions. Such operational situations are well documented, but include:
(i) Cell reversal: fuel cells occasionally are subjected to a voltage reversal (cell is forced to the opposite polarity) often caused by a temporary depletion of fuel supply to the anode. This then leads to temporary undesirable electrochemical reactions taking place in order to maintain the generation of the electrical current, such as carbon electro-oxidation at the anode which occurs at a higher potential than the oxygen reduction reaction at the cathode. In such a cell reversal situation (even for very short durations), the anode structure can be irreversibly damaged, due to oxidation of the carbon thus leading to loss of the electrocatalyst support.
(ii) Start-up/shut-down: when a fuel cell has been idle for some time it is quite possible for oxygen from the air to diffuse through the membrane from the cathode side and to displace any residual hydrogen still present in the anode side. When the cell is re-started and hydrogen is re-introduced into the anode, a mixed hydrogen/air composition will exist in the anode for a short period as a front that moves through the cell until the air is purged completely from the anode. The presence of a front that is hydrogen-rich on the inlet side and air-rich on the outlet side can set up an internal electrochemical cell within the fuel cell, such that carbon electro-oxidation is forced to occur at elevated potentials on the cathode side as the counter-reaction to oxygen reduction occurring at the outlet side of the anode. In such a start-up situation, the cathode structure can be irreversibly damaged, due to oxidation of the carbon and thus permanent degradation of the cathode catalyst layer structure can occur. A similar damaging electrochemical cell may also be set up on shut-down. Although it may be possible to limit these processes from occurring by employing system mitigation strategies, for example purging of the anode gas space with an inert gas such as nitrogen during shut-down, an MEA solution alleviates the need for these system complexities.
Solutions proposed to address the problems associated with incidences of high electrochemical potentials include employing an electrocatalyst support that is more resistant to oxidative corrosion than conventional electrocatalyst supports or incorporating an additional electrocatalyst composition that has activity for an alternative oxidation reaction that could take place at the high electrochemical potentials in preference to the damaging carbon electro-oxidation reactions, such as the oxygen evolution reaction (electrolysis of water).